In flying spot telecines, light from a Cathode Ray Tube (CRT) is used to scan information from motion picture film. Each location on the CRT corresponds to a point on the film. To make a video signal representing the information on the film, a raster is generated on the CRT. Film modulated light from the CRT is picked up by light sensors and converted to electrical video signals. Zoom is accomplished by changing the size of the CRT raster. Pans are done by applying horizontal offsets and tilts are accomplished by applying vertical offsets to the scanning raster. Flips are done by inverting the direction of the vertical or horizontal component of the raster. Rotate is accomplished by cross coupling a portion of the horizontal and vertical scans as a function of the desired angle. The telecine deflection system must quickly adapt to the frame to frame changes in scanning parameters. This requirement prevents the use of highly efficient resonant deflection systems since resonant deflection systems normally do not respond to changes at the speed required by telecines.
It is customary to use magnetic deflection to deflect the electron beam in the telecine. Today, a typical telecine has a two channel high power deflection amplifier. The two channels are substantially identical and each drive one axis of the deflection yoke. The deflection yoke is typically made up of two coils of similar inductance and sensitivity. Coil inductance of 27 .mu.H to 30 .mu.H is common. For the rotate function to perform well, it is desirable to have the horizontal and vertical channels similar in performance and both need to be fast. This is quite different than a typical television or computer monitor deflection system where the horizontal and vertical deflection channels are usually much different. This is because the vertical channel is typically low speed on a television or monitor. Typically vertical is running at about 60 Hz and horizontal is 15 kHz to 100 kHz and there is little or no rotate needed.
The voltage required across a deflection coil is a function of the coil inductance multiplied by the rate of change of the desired current through the coil. To make a raster, a triangle wave is typically generated on each scan channel. At the end of each scan line the current must change rapidly to bring the beam back to the start of the next line. This rapid change in current requires a large voltage across the coil. For standard definition telecines it is common to use a 21 kHz horizontal scanning rate with a retrace time about 8 .nu.S. However, for high definition (HDTV) telecines, since there are more horizontal scan lines (e.g., 1080 vs. 486 in standard definition NTSC systems), more lines need to be scanned per frame of video information. If the vertical rate is the same and more lines are needed, the horizontal rate needs to increase. This normally reduces the retrace time. If a frame store is used, it is possible to keep the same retrace time but at the expense of a higher video sample rate and extra bandwidth required in the video path. The deflection amplifiers need to be able to handle the extra fast retrace time required by HDTV. Without changing the deflection coil, this requires higher voltages during retrace.
The coil voltage requirements can be reduced by lowering the inductance, but the coil will be less sensitive so more current is needed. However, the increased current has an undesired side effect of adding more spot spread in the deflection process. This effectively lowers the overall telecine resolution.
Faster retrace time can be accomplished by simply increasing the power supply voltage on the deflection amplifier. However, the increase in voltage typically causes problems with power dissipation and voltage breakdown on the amplifier semiconductors.
The rate of change of the coil current during the active scan retrace is much lower than during retrace. This corresponds to lower voltage requirements during active scanning. In a linear amplifier with fixed power supplies, the power dissipation is great during active scan. It is well known that a higher voltage can be switched to the coil during retrace to decrease the retrace time while maintaining good efficiency.
U.S. Pat. No. 3,983,452 ("Brazin") and U.S. Pat. No. 4,262,235 ("Neves") describe this approach. Both these patents switch in a higher voltage during retrace. Neves uses a one shot to control the width of the high voltage pulse. This would not function well in an application, such as a telecine where the retrace pulse width would need to change based on the rate of change in current caused by user controls such as zoom, rotate, etc. Brazin's approach couples the high voltage switch to a difference amplifier which would respond better to changes in scanning. Both these approaches would not work in telecine applications where flips are used. Rotate and flip can invert the scan so the high voltage during retrace would need to have the capability of being either polarity.
U.S. Pat. No. 4,164,688 ("Cushing") describes a pair of high voltage switches, one connected to a positive high voltage source and the other to a negative high voltage source. The diodes D1 and D2 have the potential for causing crossover distortion as the current crosses over zero volts. The linear amplifier output transistors need to be able to withstand the high voltage plus the lower supply rail. This would limit the maximum high voltage used or require tradeoffs in transistor selection. Typically high power transistors optimized for linear operation and that withstand high voltages, are not available.
FIG. 2 shows a conventional grounded load deflection amplifier. Input signal 250 represents the desired deflection current for any point in time. Amplifier 201 converts the input 250 to a current through the deflection coil 210. Current sense resistor 211 provides a feedback signal 253 to amplifier 201. FIGS. 3A through 3D show the various waveforms present in the circuit. FIG. 3A shows a typical input waveform. FIG. 3B shows the voltage 253 at the sense resistor 211 representing the actual current being delivered to the coil 210. Note that the actual current waveform is different than the input waveform. This is do to the finite amount of voltage available in a practical amplifier. FIG. 3C shows the difference between input 250 and the current sense signal 253. FIG. 3D shows the voltage 252 at the coil. During retrace coil voltage 252 rapidly assumes the maximum value amplifier 201 can supply. At the end of retrace eventually it will drop down to a voltage defined by the inductance of the coil, the rate of change of the current and the value of the sense resistor. This voltage V is described by equation 1 below. The coil inductance L is in Henrys, current I in Amps, time t in Seconds and resistance R is in Ohms. ##EQU1##
If the input wave shown in FIG. 3A is inverted, all the waveforms shown in FIGS. 3B through D also will be inverted.